Semiconductor chips may be bonded onto metallic substrates such as lead frames by soldering. Specifically, power semiconductors are commonly fixed to lead frames by using soft solder. Power devices are widely used in the automotive industry due to their high thermal and electrical performance, and soft solder is generally chosen as the chip attach layer for such semiconductor packages. The soldered joint not only provides mechanical fixation of the chip, but it also ensures that heat which is produced in the semiconductor chip during operation is dissipated more efficiently than a joint comprising a non-conductive adhesive. Additionally, the soft solder provides a good electrical path for the semiconductor chips.
In cases where there is an increase in power density of the semiconductor chip, a soldered joint having a stipulated thickness is required. The layer of solder should wet the entire area of the semiconductor chip evenly. Furthermore, it should be completely free from air cavities and contamination by impurities. The solder should not protrude from the edges nor spread beyond the surface area of the bond pad, in what is known as bleeding. To achieve this, the stipulated amount of solder should be dispensed and accurately positioned onto the substrate before bonding the semiconductor chip.
Typically, soft soldering of a semiconductor chip to a lead frame entails dispensing an amount of solder onto the lead frame before positioning the chip. This may involve touching one end of a solder wire at the position of the lead frame where the chip is to be located. The lead frame is preheated to a temperature which is above the melting point of the solder so that the solder wire melts upon contact with the lead frame. A prescribed length of the wire is fed to the lead frame and it is continuously or intermittently melted onto the lead frame. The solder wire which does not melt is then pulled back leaving a droplet of solder on the lead frame. The solder droplet remains molten when the lead frame is maintained at a temperature above the melting point of the solder. Surface tension in the molten solder leads to the formation of dome-shaped droplets on the lead frame, which inhibits the formation of a flat and even solder layer under the entire chip surface when the chip is bonded onto the solder. An additional factor that affects the proper formation of an evenly-spread layer of solder is the possible lateral deviation of the solder droplet when it is being positioned on the lead frame. It is therefore essential to modify the shape of the droplets of solder to achieve a thin and evenly spread-out layer before positioning the chip.
A conventional method to modify the shape of a solder droplet utilizes spankers. The substrate carrying a solder droplet is transferred to a spanking zone where a spanker in the form of a rectangular mold is located. The rectangular mold moves downwards to contact the solder droplet and to spread it inside the spanker mold cavity to form a rectangular footprint. A spanker typically comprises a shaft which can be lifted or lowered with respect to the substrate, and a stamp or punch connected to it wherein the stamping or punching surface faces the substrate. Deviations in thickness and position of the substrate as well as possible slanting of the substrate may adversely affect the thickness and lateral distribution of the solder. Even slight deviations in the angle between the punch surface and the surface of the substrate may lead to considerable lateral displacement of solder droplets. Additionally, flattening the solder droplets by means of the spanking stamp or punch results in solder spots which may be more or less rounded at their edges and which do not conform exactly to the rectangular or square shape of the chips.
U.S. Pat. No. 6,056,184 entitled “Apparatus for Shaping Liquid Portions of Solder in Soft Soldering Semiconductor Chips” discloses a punch with a distance keeping means for shaping spherically domed liquid solder into flat solder applications to achieve an evenly distributed and correctly positioned layer of soft solder. The punch is spring mounted with respect to a shaft and slewable in all directions in relation to the axis of motion of the shaft. The movable punch allows self-adjustment in relation to the substrate such that a fixed distance from the substrate can be maintained even when the substrate is tilted. The distance keeping means affixed to the punch protrudes beyond the lower surface of the punch and is adapted to touch down onto the substrate. The distance keeping means has a rim which actively limits and centers the solder mass when it is spread over the substrate. Therefore, lateral deviations in the position of the applied solder-droplets will not affect formation of an even layer of solder under the punch surface. Furthermore, this apparatus renders it unnecessary to observe a predefined precise height or distance of the punch surface from the substrate when lowering the shaft.
Unfortunately, an uneven layer of solder may form between the chip and the substrate and in particular, the area around the edge of the chip may not be wetted by the molten solder. Moreover, when an excessive amount of molten solder is applied, solder splash may result when the mounting head presses on the chip onto the molten footprint.
It is thus important to dispense the amount of solder accurately and precisely. U.S. Pat. No. 5,878,939 entitled “Method and Apparatus for Dispensing Liquid Solder” discloses a dispenser which controls the solder wire's rate of feed to accurately regulate the amount of solder dispensed. The apparatus comprises a guide tube through which solid solder is gradually fed. The end of the tube is heated to liquefy the solder. At the same time, an adjacent zone of the guide tube is cooled to maintain a stable temperature transition in the tube. The amount of molten solder available above an outlet for the molten solder is thus controlled. This patent also discloses a spanker in the form of a molding die with a molding cavity on its underside to determine and restrict the surface wetted by the molten solder.
After spanking, the substrate bearing the rectangular footprint of molten solder is transferred to a chip mounting zone. A chip is bonded onto the rectangular solder footprint by a mounting head. The pressure on the chip is released when the mounting head moves away from the solder. The surface tension of the molten solder leads to solder flow back beneath the chip. Finally, the substrate goes through a cooling section and the layer of solder solidifies. To prevent the solder from oxidizing, chip attachment is performed under a protective or reducing atmosphere.
There are disadvantages in using the spanking method to achieve a thin and even layer of molten solder. If insufficient solder is used, especially where a large solder dot volume is required, the molten solder cannot fill up the mold cavity to form an evenly distributed footprint. This affects the bonding strength between the chip and the substrate. On the other hand, when too much solder is applied, it will lead to solder splash. Whilst this problem may be overcome by the invention under U.S. Pat. No. 5,878,939 as described above, there is an additional need to control the quality of the spanker mold fabricated so as to minimize wetting by the liquid solder which may form contaminants in the mold cavity of the spanker. Contaminants reduce the volume of the molten solder, creating voids and weakening chip adhesion and electrical conductivity. Tilted spanker molds that may result from defective fabrication will also lead to incomplete solder footprints that affect the quality of the final product. Moreover, it is difficult to achieve complex solder footprints using spanker mold technology.
It is therefore desirable to devise alternative methods of obtaining an evenly distributed layer of molten solder for soldering a semiconductor chip to a substrate to avoid the disadvantages associated with using spankers.